1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the same. More particularly, the invention relates to a vertically aligned liquid crystal display device such as of the MVA mode and to a method of manufacturing the same.
2. Description of the Related Art
As the active matrix-type liquid crystal display device (LCD), there has heretofore been widely used a liquid crystal display device of the TN (twisted nematic) mode in which a liquid crystal material having a positive dielectric anisotropy is so arranged as to be horizontal with respect to the surface of a substrate but is twisted by 90 degrees between the opposing substrates. However, the liquid crystal display device of the TN mode is accompanied by a problem of poor viewing angle characteristics, and study has been extensively forwarded in an attempt to improve viewing angle characteristics.
As a mode to substitute for the TN mode, the applicant of the present application is developing a liquid crystal display device of the MVA (multi-domain vertical alignment) mode in which the liquid crystal material having a negative dielectric anisotropy is vertically aligned and the directions of tilt of liquid crystal molecules are limited by protrusions formed on the surface of the substrate and slits formed therein when a voltage is applied. The liquid crystal display device of the MVA mode has succeeded in greatly improving the viewing angle characteristics.
A general liquid crystal display device of the MVA mode will now be described with reference to FIGS. 13A to 14. FIGS. 13A and 13B are schematic views of when the cross section of the liquid crystal display device of the MVA mode is viewed aslant. FIG. 14 is a schematic view illustrating the constitution of three pixels in the liquid crystal display device of the MVA mode and the directions of alignment of the liquid crystal molecules. In the liquid crystal display device of the MVA mode as shown in FIGS. 13A and 13B, liquid crystal molecules 108 of a liquid crystal material having a negative dielectric anisotropy are aligned nearly vertically to the surface of the substrate between two pieces of glass substrates 110 and 111. Though not illustrated, a pixel electrode connected to a thin-film transistor (TFT) is formed on one glass substrate 110 for each pixel region, and a common electrode is formed on the whole surface of the other glass substrate 111. Protrusions 120 are formed on the pixel electrode, and protrusions 121 are formed on the common electrode. The protrusions 120 and 121 are alternately arranged. Vertical alignment films that are not shown are formed on the pixel electrodes, on the common electrode and on the protrusions 120 and 121.
When the TFT is turned off and no voltage is applied to the liquid crystal molecules 108, the liquid crystal molecules 108 are aligned nearly vertically to the interface of the substrate as shown in FIG. 13A. When the TFT is turned on and a predetermined voltage is applied to the liquid crystal molecules 108, the directions of tilt of the liquid crystal molecules 108 are limited by the structure of the protrusions 120 and 121. Therefore, the liquid crystal molecules 108 are aligned in a plurality of directions as shown in FIG. 13B. For example, when the protrusions 120 and 121 are formed as shown in FIG. 14, the liquid crystal molecules 108 are aligned in four directions A, B, C and D in each pixel. In the liquid crystal display device of the MVA mode, as described above, the liquid crystal molecules 108 are arranged in a plurality of directions in each pixel when the TFT is turned on, and favorable viewing angle characteristics are obtained.
In the liquid crystal display device of the MVA mode, the directions of tilt of the liquid crystal molecules 108 are not limited by the alignment film. Therefore, the MVA mode does not require the step of alignment such as rubbing that is needed by the horizontal alignment mode as represented by the TN mode. This offers advantages from the standpoint of a process eliminating the problem of static electricity and dirt caused by rubbing and eliminating the step of washing after the alignment processing. This further offers an advantage from the standpoint of display quality without developing display shading that stems from the dispersing pre-tilt. As described above, further, the liquid crystal display device of the MVA mode can be manufactured at a decreased cost owing to a simplified manufacturing process and an improved manufacturing yield and, further, features a high display quality.
However, the liquid crystal display device of the MVA mode, too, has a serious problem that must be improved. FIG. 15A is a graph illustrating gradation viewing angle characteristics of the liquid crystal display device of the MVA mode. FIG. 15B is a diagram illustrating the polar angle and the azimuth angle. As shown in FIGS. 15A and 15B, when the transmittance-voltage characteristics (T-V characteristics) in a tilted direction (polar angle of 60°, azimuth angle of 45°) are compared with the T-V characteristics in the front direction (polar angle of 0°), there exist a gradation region where the transmittance increases in a tilted direction with respect to the transmittance in the front direction, and a gradation region where the transmittance decreases with respect to the transmittance in the front direction. In the liquid crystal display device of the MVA mode, therefore, there occurs a problem in that the chromaticity that is viewed from the front direction deviates from the chromaticity that is viewed from a tilted direction.    Patent document 1: Japanese Patent No. 3520376.    Patent document 2: JP-A-2000-347174    Patent document 3: JP-A-2002-107730